Degasifying apparatus

ABSTRACT

Degasifying apparatus for eliminating gases, such as ambient air, from fluids, such as oil, said apparatus consisting of at least one permeable membrane ( 26 ) which lets the gas to be eliminated from the fluid ( 22 ) penetrate therethrough and retains the liquid moiety of the fluid.

The invention relates to a degasifying apparatus for eliminating gases such as ambient air from fluids such as oil.

Air may be present in dissolved and in undissolved form in pressure fluids such as hydraulic fluids. Although dissolved air is not visible in the fluid or oil, it is always present to a certain extent. Undissolved air is not always present in the fluid or oil, but when it is, it is visible as a second phase. Although the properties of pressure fluids, such as viscosity, compression modulus, and lubricating capacity, are substantially impaired by undissolved air, dissolved air in also affects certain properties of the pressure fluid, for example ageing behavior and cavitation tendency.

The maximum amount of air that can be dissolved in the fluid concerned is determined by the saturation curve of the fluid concerned. However, in principle, solubility increases with increasing pressure and to a lesser extent is also dependent on temperature. Because pressures and temperatures in hydraulic systems vary over time as well as according to location, a drop in pressure and consequent lowering of the solubility limit can result in air dissolving out during operation. As a result, the formation of a second phase (bubbles) and thus damage, for example by flow cavitation, can be expected, especially in areas of low static pressure, such as control cross sections of valves. Because the outgassing rate is greater than the dissolution rate, resulting air bubbles remain even when the pressure in the fluid increases again, thereby altering the properties of the pressure fluid on the one hand and possibly resulting in damages due to cavitation erosion upon a subsequent pressure increase on the other hand. In order to ensure reliable operation of hydraulic systems, it is therefore necessary to take measures for degasifying the pressure fluid. Vacuum evaporation is the process currently employed for achieving a particularly effective degasification, in which the degassed fluid contains only a small residual contamination. Although this has the advantage that water is simultaneously eliminated, the very high energy requirement and the elaborate construction of the apparatus are disadvantageous. In addition there is usually an undesired heating of the fluid, and it is not possible to integrate the degasifying apparatus in a main fluid flow of the fluid system.

In view of these problems, the object of the invention is that of providing a simple and cost-effectively operable degasifying apparatus for fluids such as hydraulic fluid.

According to the invention, this object is achieved by a degasifying apparatus having the features of claim 1 in its entirety.

Accordingly, the invention provides a permeation process for desgasifying, which takes place through a membrane. For separation, use is made of a dense membrane without pores, through which a diffusion process takes place. In comparison to other desgasifying techniques such as vacuum evaporation, the diffusion process can be carried out with very little energy use, as only a partial pressure gradient at the membrane is needed as a driving force. For desgasifying pressure fluids such as hydraulic fluid in hydraulic systems, the available pressure in the system can be used to increase the partial pressure differential. By means of the apparatus according to the invention, in which the degasifying is effected by allowing the fluid to flow over a membrane, the degasifying process can be advantageously carried out within the system, namely in a bypass of the pressure system as well as in a main volume flow of the system.

The permeable membrane used in the apparatus according to the invention can comprise a silicone material, and can preferably be composed entirely of silicone.

In particularly advantageous exemplary embodiments, a support body composed of a wire gauze, a sintered metal, a ceramic, or other structure, each having passages or pores, of which the free cross sections permit a gas passage, is provided for supporting the permeable membrane.

The permeable membrane with its support body can separate a fluid side from a gas side in a container, or the membrane can comprise a fluid guide. In this manner, the degasifying process can take place on a volume flow of a hydraulic system, flowing through a conduit encased by the tubular support body, against the inside of which the permeable membrane that separates the free cross section of the conduit from the exterior support body in a fluid-tight manner rests.

In advantageous exemplary embodiments, the permeation coefficient Q of the silicone membrane is 200 to 600×10⁻¹⁷ m²/s/Pa, preferably in the range of values from 300 to 400×10⁻¹⁷ m²/s/Pa, and particularly preferably in the range of values from 370 to 380×10⁻¹⁷ m²/s/Pa. Silopren®LSR 2640 is a commercially available silicone rubber that can be advantageously used as a silicone membrane.

With particular advantage, the degasifying apparatus can be integrated in a hydraulic system comprising a low pressure and a high pressure hydraulic accumulator, which are hooked up as an energy recovery device on the gas side to a hydrostatic drive, which preferably enables a four quadrant mode of operation, wherein the permeable membrane degasifies the fluid on the low pressure side of the energy recovery device.

With particular advantage it can be arranged such that the low pressure side of the energy recovery device is hooked up to the container with the permeable membrane, on the fluid inlet side of said container, and that the fluid outlet side of the container is hooked up to a transport device in the form of a spring-loaded differential piston pump or a Venturi nozzle. The degasifying apparatus thus forms a bypass to the low-pressure side of the energy recovery device.

In order to actuate the spring-loaded differential pressure pump, the larger piston face thereof can be subjected to the pressure of the low-pressure side of the low-pressure accumulator. The system pressure of the low-pressure side thus supplies the drive for the transport device.

The gas side of the container can have ambient pressure or it can be hooked up to a suction device, which increases the partial pressure differential on the membrane.

In particularly advantageous fashion, this suction device can have another spring-loaded differential piston pump, the larger piston face of which can be subjected to the pressure of the low pressure side of the low pressure accumulator for actuation and the movement of which is pressure-synchronized with the first differential piston pump. The system pressure of the low-pressure side thus also supplies the drive for the suction device.

The invention is explained in detail below, with reference to exemplary embodiments illustrated in the drawings, wherein:

FIG. 1 shows a schematically simplified and symbolically depicted hydraulic circuit diagram of a first exemplary embodiment of the degasifying apparatus of the invention;

FIGS. 2-4 show the circuit diagram for a second, third, and fourth exemplary embodiment of the invention, respectively, illustrated in a manner corresponding to that of FIG. 1, and

FIG. 5 shows a longitudinal section of a pipe section of a fluid guide with a fifth exemplary embodiment of the degasifying apparatus according to the invention.

With reference to FIGS. 1-4 of the drawings, the invention is explained with examples, in which the degasifying apparatus is a component of the energy recovery device in a hydrostatic drive system. As disclosed in Document DE 10 2008 062 836 B3, which shows an example of such a drive system, such systems have, in combination with a hydropneumatic high-pressure accumulator 2 and a hydropneumatic low-pressure accumulator 4, a swivel-mounted four quadrants drive 6, which is coupled to a wheel drive via a shaft 8. The high-pressure accumulator 2 and the low-pressure accumulator 4 are connected by their oil sides 10 and 12, respectively, to the four-quadrants drive 6, which charges the high pressure accumulator 2 during braking processes in the pump mode for storing braking energy, and which for acceleration processes functions as a hydraulic motor, which is driven by the pressure fluid of the high-pressure accumulator and from which depressurized fluid arrives at the low-pressure accumulator 4. The latter in turn supplies the fluid volume for the braking energy-storing charging processes of the high pressure accumulator 2. In operation, the pressure in the low-pressure accumulator 4 varies between ca. 3 bar and ca. 15 bar during these working cycles of the energy recovery devices.

In these exemplary embodiments, the associated degasifying apparatus according to the invention has a container 14, to which the fluid 22 to be degasified can be conducted from the low pressure side 16 of the energy recovery device via a line 18, which opens into the bottom of the container 14 via a pressure relief valve 20. From the container 14, the fluid 22 can be returned to the low-pressure side 16 via another line 24. In this arrangement, the degasifying apparatus forms a bypass to the low-pressure side 16 of the associated system.

In the container 14, a membrane 26 separates the chamber containing the fluid 22 to be degasified from a chamber 28 that receives the gaseous phase that has passed through the membrane 26 by diffusion. From the top side of the chamber 28, this air that has been degassed from the fluid 22 passes to the surroundings 32 via a line 29 and via a venting filter 30. The pressure relief valve 20 is set to a value at which the acting pressure of the low-pressure side 16 is reduced to a value that corresponds to the partial pressure gradient at the membrane 26 desired for the diffusion process, in other words the pressure gradient relative to the ambient pressure prevailing in the chamber 28.

A transport device 34 is situated in the line 24 that is provided for the return flow of the fluid 22 from the container 14 to the low-pressure side. In the example of FIG. 1, this device has a differential piston pump 36, the smaller piston face 40 of its differential piston 38 being guided in a displacement chamber 42 for the fluid 22 to be transported, whereas the larger piston face 44 is guided in a drive chamber 46. The displacement chamber 42 is connected to the bottom of the container 14 via a check valve 48 acting as a suction valve and to the line 24 leading back to the low pressure side 16 via a check valve 50 acting as a pressure valve. A compression spring 52 pretensions the differential piston 38 in the direction for reducing the volume of the displacement chamber 42. The pressure of the low-pressure side 16 of the system is applied to the drive chamber 46 and thus to the larger piston face 44 via a branch 54. The action of the compression spring 52 is selected such that during operation phases, in which the pressure of the low pressure side 16 is below a threshold value, the compression spring 52 moves the piston 38 for reducing the volume of the displacement chamber 42, whereas in operation phases in which the pressure of the low pressure side 16 exceeds the given value, the pressure in the drive chamber 46 moves the piston 38 against the action of the compression spring 52, resulting in a working cycle of the piston pump 36 for alternating operation phases, causing the fluid 22 to be degassed to flow through the container 14.

The exemplary embodiment of FIG. 2 differs from the first example only in that air, which is displaced by movements of the differential piston 38 from the chamber 56 of the piston pump 36 containing the compression spring 52 and which is possibly contaminated with lubricants, does not reach the surroundings without being filtered. Hence a line 58 is hooked up to this chamber 56, which line is connected to the line 29 that vents the degassed air from the chamber 28 of the container 14. The line 58 is thus connected to the line 29 between two check valves 62 and 64, which each open to the the surroundings 32 when actuated by pressure. In this arrangement, the differential piston pump 36 not only forms the transport device 34 for transporting the fluid, but also simultaneously forms a suction device that creates a negative pressure in the line 58 during piston movements in which the volume of the chamber 56 containing the compression spring 52 increases, which negative pressure also acts, via the check valve 62 opening thereunder and via the line 29, in the chamber 28 of the container 14 and thus on the membrane 26.

Besides the differential piston pump 36 that forms the transport device 34, a second differential piston pump 70 is provided as a suction device 68 in the exemplary embodiment of FIG. 3. As is the case with the first piston pump 36, the smaller piston face 74 of the differential piston 72 acts in a displacement chamber 76, whereas the larger piston face 76 moves in a drive chamber 78. The latter, like the drive chamber 46 of the first piston pump 36, is subjected to the pressure of the lower pressure side 16. As is the case with the first piston pump 36, the piston 72 is pretensioned by a compression spring 80 for a movement that reduces the volume of the drive chamber 78. In this arrangement, the second piston pump 70 with its displacement chamber 76, which is connected via a suction line 82 to the line 29 at a point situated between the check valves 62 and 64, acts as a suction pump, creating a negative pressure in the line 29 and thus on the membrane 26 located in the container 14.

FIG. 4 shows an exemplary embodiment in which the transport device 84 has a Venturi nozzle 86, through which, in a bypass to the four quadrants drive 6, a volume flow flows from the oil side 10 of the high-pressure accumulator 2 to the low-pressure side 16, wherein the volume flow is limited to a low intensity by a pressure reducing valve 88, namely to a value that suffices for creating a negative pressure at the suction port 90 of the Venturi nozzle 86, leading to an outflow of degassed fluid 22 via the line 24 to the low pressure side 16.

The exemplary embodiment of FIG. 5 enables a degasifying within a fluid guide of a (not shown) fluid system. In this case, the membrane 26 forms the fluid-tight casing of the interior of a pipe section 92, the outer pipe wall of which forms a support body 94 for the membrane 26, which on the inside is acted on by the pressure of the fluid 22 to be degassed. In this example, this support body is formed by a sintered metal, which has a porosity permitting air to permeate. Instead of a sintered metal, a wire gauze, a porous ceramic material, or any other structure that has passages or pores for gas penetration could be provided as a material for the support body 94.

As in the examples described above, the membrane 26 can advantageously be made of a silicone material having a thickness of 1 mm to 2 mm, for example Silopren®LSR 2640, wherein the thickness of the material is selected such that the permeation coefficient Q lies in an advantageous range of values, preferably in the range of between 370 and 380×10⁻¹⁷ m²/s/Pa. 

1. A degasifying apparatus for eliminating gases, such as ambient air, from fluids, such as oil, consisting of at least one permeable membrane (26), which allows the gas to be eliminated from the fluid (22) to pass through and retains the liquid portion of the fluid.
 2. The degasifying apparatus according to claim 1, characterized in that the permeable membrane (26) comprises a silicone material, preferably consists entirely of silicone.
 3. The degasifying apparatus according to claim 1, characterized in that the permeable membrane (26) rests against a support body (94) consisting of a wire gauze, a sintered metal, a plastic material, or a ceramic; all of which having passages or pores, of which the free cross sections permit gas to permeate (penetrate).
 4. The degasifying apparatus according to claim 1, characterized in that the permeable membrane (26) with its support body (94) separates a fluid side from a gas side (28) in a container (14) or comprises a fluid guide (92).
 5. The degasifying apparatus according to claim 1, characterized in that the permeation coefficient Q of the silicone membrane (26) is between 200 and 600×10-17 m2/s/Pa, is preferably in the range of values between 300 and 400×10-17 m2/s/Pa, particularly preferably in the range of values between 370 and 380×10-17 m2/s/Pa.
 6. The degasifying apparatus according to claim 1, having a low-pressure accumulator (4) and a high-pressure accumulator (2), which as an energy recovery device are hooked up on the gas side to a hydrostatic drive (6, 8), which preferably enables operation in a four quadrant mode, wherein the permeable membrane (26) degasifies the fluid (22) on the low pressure side (16) of the energy recovery device.
 7. The degasifying apparatus according to claim 1, characterized in that the low pressure side (16) of the energy recovery device is hooked up to the container (14) with the permeable membrane (26), on the fluid inlet side of said container, and that the fluid outlet side (29) of the container (14) is hooked up to a transport device (34, 84) in the form of a spring-loaded differential piston pump (36) or a Venturi nozzle (86).
 8. The degasifying apparatus according to claim 1, characterized in that the spring-loaded differential piston pump (36) can be subjected, on its larger piston face (44), to the pressure of the low-pressure side (16) of the low-pressure accumulator (4) for its drive.
 9. The degasifying apparatus according to claim 1, characterized in that the gas side (28) of the container (14) has ambient pressure (32) or is hooked up to a suction device (56, 58; 68; 84).
 10. The degasifying apparatus according to claim 1, characterized in that the suction device (68) has another spring-loaded differential piston pump (70), which can be subjected, on its larger piston face (76), to the pressure of the low pressure side (16) of the low pressure accumulator (4) for its drive and which is pressure-synchronized with the first differential piston pump (36) in its movement. 